Well pipe is made up by supporting a lower pipe section ("joint") in the well and then threading an upper joint onto it by means of a fluid-driven power tongs. The pipe assembly is lowered as new joints are added, down to depths of several miles. Threaded well joint connections, in order to seal properly and to have maximum tensile strength, must be accurately tightened ("made-up" in the trade) to a design torgue ("make-up torque") specified by the pipe manufacturer. The design torque must not be exceeded, since galling or breakage of the pipe threads may result. This is particularly true with pipe joint materials chosen for considerations other than strength, e.g. corrosion resistance and impermeability. Such materials are not only relatively soft--they can be quite expensive. In one recent case, 1000 joints (each thirty-three feet long) were removed from a well. Every joint had thread damage due to overtorquing and was considered scrap. This was pipe originally costing $2500 per joint. The importance of controlling the torque applied by the power tongs to the pipe can thus be appreciated, and in fact it is a requirement on many jobs that a running record of maximum torque at each joint be kept. (Various systems exist for making torque records during make-up, including applicant's system described in co-pending applications Ser. Nos. 487,048 and 526,611.) Despite the existence of accurate torque recording systems, improper torquing continues to occur. The industry still seeks a system that will positively prevent thread damage from overtorquing.
A second consideration is that thread damage can result not only from overtorquing but also from pipe misalignment. When the hoist supporting the upper end of a joint undergoes large lateral excursion occasioned perhaps by high winds, misalignment sufficient to cause cross-threading can occur. Once the threads are crossed, not much torque is required to ruin the threads. If the crossed thread is not detected, a leaky connection can result even though the proper torque is applied, since in that instance torque may not be an adequate indicator of sealing force.
The crossed thread problem is aggravated by violent or jerky movement of the tongs when power is first applied. The tongs frequently do not work smoothly--and are hard to control--at very low speeds. Also, the snub line, initially slack, tends to snap tight when power is first applied. These conditions make it difficult to control and/or record torque at the instant tongs operation begins, so that thread damage can occur even if a low-level torque limiter is used.
Even if the threads are not crossed, misalignment of the pipes can cause binding of the threads sufficient to produce galling as the pipe is rotated.
I have found that the above problems can be overcome by substantially increasing the overall gear reduction ratio within the tongs, for example, by a factor of five, so that the tongs jaw speed is correspondingly reduced, avoiding the problems of irregular start-up. This speed reduction is advantageously combined with a two-stage torque limiter system for (a) preventing the application of substantial torque during the initial phase of make-up and (b) limiting the maximum torque that the tongs can produce at the final make-up stage. Such a system is described in my co-pending application Ser. No. 629,421, supra.
The present invention is particularly useful for assembling connections of the type shown in U.S. Pat. No. 3,359,013. This type of connection has one or more annular shoulders associated with each thread, for engaging a corresponding shoulder on the mating piece. The threads themselves, being of a non-interference type, do not provide sealing, which occurs entirely at the contacting shoulders. During assembly, the pipe can be rotated by hand until shoulder contact occurs; thereafter only minor rotation, perhaps one-eighth turn, is needed fully to make up the connection. During this stage the required torque rises rapidly from hand-tight to, for example, 2000 ft.-lbs. Comparative charts of torque T vs. turns N for conventional and shouldered threads are shown in FIGS. 3a and 3b. Plainly, the more rapid torque increase rate of the shouldered connection calls for a torque controller having fast response.